Corneal Problems
The cornea is the outermost layer of the eye and problems that affect it can cause double vision. Astigmatism is one of such problems where the cornea is irregularly shaped, resulting in monocular double vision. A rare type of progressive astigmatism (keratoconus) [5] which is marked by distortion of the surface of the cornea into a cone shape can also result in double vision when looking with just one eye.
Other problems of the cornea that can result in double vision include dry eyes, corneal scarring due to injury or infection, and shingles.
Conclusion
Double vision is a treatable condition which can usually be fixed once the cause has been established. Sudden onset of prolonged double vision can have potentially life-threatening causes. See a doctor to find out the cause of your diplopia. Diagnosis may involve blood tests, magnetic resonance imaging (MRI). You may need to see a specialist depending on the situation.
References:
[1] https://stanfordhealthcare.org/medical-conditions/eyes-and-vision/double-vision.html
[2] https://stanfordhealthcare.org/medical-conditions/eyes-and-vision/thyroid-eye-disease.html
[3] https://www.mayoclinic.org/diseases-conditions/multiple-sclerosis/symptoms-causes/syc-20350269
[4] https://www.healthline.com/health/myasthenia-gravis
[5] https://www.webmd.com/eye-health/eye-health-keratoconus